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need 12 volts at 30 amps under hood

Discussion in 'Gen 3 Prius Technical Discussion' started by Seymour1, Oct 7, 2024.

  1. Seymour1

    Seymour1 Junior Member

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    I want to add a loud horn. The one I bought came with a 30 amp fuse. The wire and fuse going to the OEM horn is likely too weak. I am looking for a heavy wire with 12 volts, always hot, and fused at 30A or more located under the hood. My Haynes manual is not much help. Any help will be appreciated.
    (The horn is a motor-driven air horn.)
     
    #1 Seymour1, Oct 7, 2024
  2. Tombukt2

    Tombukt2 Senior Member

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    The factory horn wire is going to be your signal wire That's going to activate a relay wherever you put it in the relay will pick up 12 volts they off the battery with an ample size piece of wire which will probably match the wire that you're horn already has going to it and then off the relay when that wire gets energized it goes to the positive wire on your horn and energizes the horn all with the factory wire only thing being added is the relay that's going to open a contact that's going to allow you to feed that contact with a 30 amp sized wire and then power your 30 amp sized wire that goes right to your horn and now you have a blaring horn apparently we shall see. All your original horn wires going to do is open a relay what the relay opens to is the fat wire that you put on the battery or somewhere under the hood pick up near the fuse box probably and then the new horn wire plugs into the other side of the relay so when your factory horn wire is energized by you pushing the button it opens the relay lets the voltage from the fat wire you now have connected to one of the fat wires near the fuse box and opens that circuit and plays the horn on the 30 amp wire you'll see some fat wires up there around the fuse box you can probably tap your 30 amp horn off of without any problem your horns never going to be anything that's used continuously generally speaking
     
    #2 Tombukt2, Oct 7, 2024
  3. PriusCamper

    PriusCamper Senior Member

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    Just gotta run a new wire direct from your 12v battery terminal... Then follow what Tombukt2 is saying about the relay wire.
     
    #3 PriusCamper, Oct 7, 2024
  4. Seymour1

    Seymour1 Junior Member

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    I am not sure I follow you.
    I will use the thin wire going to the weak horn to operate the new relay that came with the new horn. That small horn wire (too small to carry 20-25 amps) will connect to the new relay coil. Then I will connect one side of the new relay contact with a heavy duty (>AWG 14) wire going to a high capacity 12 volts. The other contact of the new relay will connect with heavy wire to the new horn.
    So, what I need to find is a wire which will supply 12v @ 20-25 amps and is always hot and is under the hood. If there is not one under the hood (and easy to access), I will need to run a wire through the firewall from under the dash. (Not the best way)
    Maybe this is what you were saying? Do you have a wiring diagram of the 12v wires in the fuse block under the hood? I could connect to an unfused 12 volts since I will be adding the new 30A fuse.
    The new horn is crazy loud! I need one for my motorcycle.
     
    #4 Seymour1, Oct 7, 2024
  5. xliderider

    xliderider Senior Member

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    There's two open 1/4 inch terminals that are always hot in the engine fusebox. In this picture, there are two red wires running to them for my foglights and FIAMM horns.

    Put an inline fuse on the wire and you will be good to go. It is essentially battery power. 120510_1487566341552.jpg

    SM-G781V ?
     
    #5 xliderider, Oct 7, 2024
    PriusCamper likes this.
  6. Seymour1

    Seymour1 Junior Member

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    Thank you. Thank you. This will be easier than I thought. This is perfect. I plan to also use a fiamm horn. I will arrange the circuit so that the loud horn only blows if I have my foot on the brake. Otherwise, the quiet horn blows. The circuit needs a diode in series with the new relay. If you want, I will send you the schematic. It does require that you run a brake switch wire through the firewall. (not that hard to do) 2024-10-08_18-57-11.jpg
     
    #6 Seymour1, Oct 8, 2024
  7. xliderider

    xliderider Senior Member

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    No thanks for the schematic, I actually want my louder FIAMM horns to work at all times, especially when my foot is off the brake pedal.

    SM-G781V ?
     
    #7 xliderider, Oct 8, 2024
  8. Seymour1

    Seymour1 Junior Member

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    When I pull into a friend's driveway to pick him up for our carpool, I want to send a gentle polite message.
    If someone rudely pulls out in front of me, I want to send a not-so-gentle-message. And, I will already have my foot on the brake. The horn is very loud.
    Mainly, I thought it would be interesting to have the versitility.
     
    #8 Seymour1, Oct 10, 2024
    jonsey1886 likes this.
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